The present invention relates to the disposal of plant waste, and particularly coffee grounds. Plant waste here referring to vegetative matter from biological plants. Those skilled in the art have known, for numerous years, how to manufacture a water-soluble powder, particularly by freeze-drying, commonly called instant coffee. It goes without saying that after having extracted from the coffee those constituents that form the water-soluble powder, waste remains, in the form of grounds, which has to be disposed of. Several solutions can be envisaged. It is, for example, possible to make the grounds into compost. This takes a certain amount of time and requires large storage areas. It is also possible to burn them. In such case, they have to be dried, either in a furnace, which consumes energy, or in the open air, which takes time, since coffee grounds contain between 17 and 20% water. Consequently, space is required for storage.